1. Field of the Invention
The present invention relates to an apparatus for testing a clamp screw consisting of a bolt and a nut, particularly an apparatus for testing performance of a clamp screw in its elastic and plastic regions. The apparatus enables the measurement of any kind of required performance of a clamp screw in its elastic and plastic regions in order to permit tightening the clamp screw in the plastic region as well as in the conventional elastic region.
2. Related Art Statement
In general, screw parts such as bolts and nuts have been tested for quality certification by use of material tension testing machines and screw clamp testing machines, and there are measured various performance criteria for a clamping screw such as clamping torque, bearing torque, thread torque, axial tension and the twisting and revolution angle of a bolt and nut in the elastic region.
However, conventional material tension testing machines have a disadvantage in that elongation generated by clamping a bolt cannot precisely be measured. Further, conventinal screw clamp testing machines have such disadvantage that the elongation and twist angle of a clamped bolt up to break down cannot be measured, and in the actual use of a clamp screw, in external force will be applied to the clamp screw after it is clamped, but the testing machine cannot perform a test corresponding to the external force which will be applied to the clamped screw.
On the other hand, recent technical improvements are directed to the provision of highly precise and compact machines, and this has resulted in a requirement for highly reliable and small clamp screws which are inevitable to assemble the machineries.
That is, conventional clamp screws are designed for clamping in the elastic region so that even in the case of loading with an external force, the axial tension subjected by to whom the screw parts such as bolts and nuts are subjected does not exceed the limit of elasticity with a sufficient allowance. Therefor its working efficiency in strength is limited to 40-50%, which is a redundant design. Further, as to reliability, the conventional clamp screw designed for clamping in the elastic region has the initial clamp force design value which has a dispersed large difference from an actual value. On the contrary, the plastic region clamping of a clamp screw is to clamp over the yield point of the screw parts, so that its use efficiency in strength reaches 80-90%, screw parts can be miniaturized as compared with the elastic region clamping, and dispersion of the clamp force can be more reduced more than in the case of elastic region clamping. Thus, improvement of various performances can be expected.
However, the behavior of a clamp screw in the case of clamping in the plastic region is still unsolved or unknown, and there are many uncertain factors involved when precisely clamping in the plastic region. In particularly, the behavior of a screw when it is subjected to an external force after being clamped is not well-known in spite of its being an extremely important problem.